my %old_lookup;
for my $leader (keys %old) {
   $old_lookup{$_} = $leader for @{ $old{$leader} };
}

##</code><code>##

   Bill => [],

##</code><code>##

for my $newtl (sort keys %new) {
   my %needtomeet;
   for my $rep (@{ $new{$newtl} }) {
      my $oldtl = $old_lookup{$rep};
      push @{$needtomeet{$oldtl}},$rep if $oldtl;

      yadda yadda ...